The quantities and types of organic waste being generated have grown significantly in recent years. Many of these organic wastes are hazardous in that they present significant environmental anti health hazards. Hazardous wastes previously disposed of by unacceptable methods continue to be discovered in various waste sites. Improved methods for treatment and disposal of these hazardous wastes are required to meet environmental standards and to treat these wastes in a cost-effective manner.
Recently, there have been increased research efforts aimed at improving the method used to remediate hazardous materials at problem waste sites. For wastes containing hazardous organics and water, it is desirable to destroy the organics so the water can be reused or discharged to ground or surface water streams. Thermal technologies currently being developed and employed for destruction of organic material includes incineration, wet air oxidation and supercritical oxidation.
Several commercial treatment technologies exist for treating low organic content water streams (&lt;1000 ppm) by using conventional water purification methods and for treating high concentration streams (&gt;10%) by various costly methods, including solvent extraction and incineration. However, there exists the need to be able to treat concentration ranges not presently commercially treatable (1-5%), as well as other concentration ranges.
Conversion of some waste organic materials, such as contained in biomass materials, to low-Btu fuel gas, carbon monoxide and hydrogen by pyrolysis and substoichiometric burning at 500.degree. C. to 800.degree. C. and about one to ten atmospheres is well known. Studies undertaken to optimize such processes have demonstrated that high temperatures with or without catalysts are required to minimize tar and char formation in the reaction. Recent interest in organic conversion has been aimed at production of a medium-Btu fuel gas through the use of a steam and/or oxygen gasification environment. These processes would produce a cleaner carbon monoxide/hydrogen gas mixture which would be used for synthesis of methane as well as other products.
U.S. Pat. No. 4,239,499 to Pfefferle discloses a process for converting a stream containing 25% or more methanol into a product gas. However, this process has certain limitations in regard to total water content of the feedstock, and requires vapor phase reactions. The process as disclosed is limited to a maximum of 1 mole of methanol to 1.5 moles of water due to potential catalytic deactivation, as discussed at column 5, lines 40-49. Another limitation is the economics of energy required to run the process. The amount of energy required to vaporize a methanol/water stream is about 60% more than to use a liquid stream. Running a vaporized stream at 350.degree. C. requires 1080 Btu/lb. With the concentration of organic in water being held constant, for a liquid stream at 350.degree. C., the amount of energy required is 690 Btu. This approach results in a difference of 390 Btu/lb, and with 8 lb/gallon results in an energy savings of about 3100 Btu per gallon of waste stream.
U.S. Pat. No. 4,113,446 to Modell et al. discloses a process for converting organic material to a product gas by reaction with water at or above the critical temperature of water and at or above the critical pressure of water to achieve the critical density of water. The reaction is indicated to occur regardless of the presence of a catalyst, presumably because of the critical conditions. However, this process also suffers from limitations in regard to the amount of methane produced. This is exemplified by the run with hexanoic acid in which less than one percent of methane was produced from approximately a 2% solution. In view of the low amount of methane production, the energy economics of the system appear very unfavorable. For an example, taking a 10% organic in water solution, the amount of energy required to heat and pressurize the system as disclosed takes about 880 Btu/lb of solution to heat to the critical density of water. However, the gas produced will have a fuel value of only about 40 Btu.